x + 300 = 200 x 2 

x + 300 = 400

x = 400 - 300

x = 100

tk mk nha

X + 300 = 200 X 2

X + 300 = 400

X           = 400 - 300

X            = 100

X + 300 = 200 x 2

X + 300 = 400

X = 400 - 300

X = 100

Ai k mk

Mk k lai

300+x=500.

        x=500-300

        x=200

x = 200

x=500-300

x=200

X = 300 : 3

X =100 :5

X = 300- 20

X = 280

x+8=300

=> x=300-8=292

Vậy x=292

x+5*3=300

x+15=300

x=300-15

x=285

-k cj

\(x⋮12;28;21\)
\(x\in BC\left(12;28;21\right)\)mà \(BCNN\left(12;28;21\right)=84\)
\(\Rightarrow x\in\left\{0;84;168;252;336...\right\}\) mặt khác \(150< x< 300\)
\(\Rightarrow x=252\)

\(x⋮12;28;21;\)

\(\Rightarrow x\in B_{12;28;21}\)

\(B_{12}=\left\{156;168;180;192;204;216;228;240;252;264;276;288\right\}\)

\(B_{28}=\left\{168;196;224;252;280\right\}\)

\(B_{21}=\left\{168;189;210;231;252;273;294\right\}\)

\(\Rightarrow x\in\left\{168;252;\right\}\)  ( thỏa mãn \(150< x< 300\)

=> [ (x - 300)² - 280 ] . 2 = 5938

=> (x - 300)² - 280 = 2969

=> (x - 300)² = 3249

=> x - 300 = 57 => x = 357

hoặc x - 300 = -57 => x = 243

Vậy x = 357 ; x = 

X x 3=123

x     = 123 :3

x     = 41

Suy ra x =41

300 : x =60

        x = 300:60

        x = 5

Suy ra x =5

ta có

x chia hết cho 12 và 21

=> x\(\in\)BC(12,21)

12 = 2².3

21 = 3.7

BCNN(12,21)=2².3.7=84

BC(12,21) = B(84) = { 0 ; 84 ; 168 ; 252 ; 336 ; ...}

Mà 150 < x < 300 nên x = { 168 ; 252 }